1. Field of the Invention
This present invention relates to a method for producing high yields of high-purity carbon nanostructures having uniform average widths narrower than conventional carbon nanostructures. The nanostructures are produced from unsupported catalytic metal powders. A dispersing agent, such as sodium chloride, is blended with the catalytic metal powders prior to milling to the desired catalytic size to prevent the powder particles from sintering.
2. Description of Related Art
Nanostructure materials, particularly carbon nanostructure materials, are quickly gaining importance for various potential commercial applications. Such applications include their use to store hydrogen, to serve as catalyst supports, to be useful in various batteries, and as reinforcing components for polymeric composites. U.S. Pat. Nos. 5,149,584 and 5,618,875 to Baker et al. teach carbon nanofibers as reinforcing components in polymer reinforced composites. The carbon nanofibers can either be used as is, or as part of a structure comprised of carbon fibers having carbon nanofibers grown therefrom.
U.S. Pat. No. 5,413,866 to Baker et al. teaches carbon nanostructures characterized as having: (i) a surface area from about 50 m2/g to 800 m2/g; (ii) an electrical resistivity from about 0.3 μohm·m to 0.8 μohm·m; (iii) a crystallinity from about 5% to about 100%; (iv) a length from about 1 μm to about 100 μm; and (v) a shape that is selected from the group consisting of branched, spiral, and helical. These carbon nanostructures are taught as being prepared by depositing a catalyst containing at least one Group IB metal and at least one other metal on a suitable refractory support, then subjecting the catalyst-treated support to a carbon-containing gas at a temperature from the decomposition temperature of the carbon-containing gas to the deactivation temperature of the catalyst.
U.S. Pat. No. 5,458,784 also to Baker et al. teaches the use of the carbon nanostructures of U.S. Pat. No. 5,413,866 for removing contaminants from aqueous and gaseous steams; and U.S. Pat. Nos. 5,653,951 and 6,159,538 to Rodriguez et al. disclose and claim methods of incorporating hydrogen into layered nanostructure materials characterized as having: (i) crystalline regions; (ii) interstices within the crystalline regions which interstices are from about 0.335 nm to 0.67 nm, and (iii) surfaces of said nanostructure which define the interstices, which surfaces have chemisorption properties with respect to hydrogen. All of the above referenced U.S. patents are incorporated herein by reference.
Carbon nanostructures, particularly carbon nanofibers, are typically produced by growing them from suitable supported or unsupported powdered metal catalysts at elevated temperatures, in the presence of hydrogen and an effective decomposing carbon-containing compound. Typically, the carbon-containing compound is selected from CO, methane, ethane, ethylene, acetylene, propane, propylene, butane, butene, butadiene, pentane, etc. While such a method is currently used to produce carbon nanostructures in substantial yields, the width of the nanostructures is difficult to control. Narrow width nanostructures are desirable. For example, the average width of a carbon nanostructure is dependent on the average size of the metal catalytic particle from which it was grown. This size typically ranges from about 25 to 450 nm.
One attempt to overcome this shortcoming of controlling carbon nanostructure width was to disperse catalytic metal particles over a suitable substrate, such as an amorphous carbon film, in order to produce carbon nanostructures having a more uniform narrower width. This was achieved to some degree since a more uniform catalyst particle size dispersion was achieved. Although the resulting carbon nanostructures resulting from such a method were found to have an average width about half that of those produced by more conventional techniques at that time, the yield of nanostructures was vastly reduced and unacceptable. In addition, the support material becomes an added impurity that should be avoided when such a method is used.
Thus, there is a need in the art for methods for producing high yields of carbon nanostructures, especially carbon nanofibers, having a substantially uniform narrow width.